1. Field of the Invention
This invention relates to an image forming apparatus including a belt member, which can be brought into and out of contact with a rotating body and is steering controlled.
2. Description of the Related Art
There has been put into practical use an image forming apparatus including a belt member (intermediate transfer belt, recording material conveying belt, or a transfer belt), which can be brought into and out of contact with a rotating body and is steering controlled.
Japanese Patent Application Laid-Open (JP-A) No 2001-249519 discloses a tandem full-color printer in which a belt member is in press-contact with a plurality of image bearing members having different developing colors to form a full-color image. In this full-color printer, some image bearing members and the belt member are spaced from each other by movement of a portion of the belt member (a stretched surface of the belt member). Namely, in a full-color mode where four toner images are superposed, the belt member is abutted against the plurality of image bearing members to transfer the toner images of the respective colors; meanwhile, in a monochrome mode of forming a monochrome image, the belt member is spaced from an unused image bearing member. In this constitution, since only the belt member is moved, the constitution is easier and more preferable, compared with the conventional constitution in which the plurality of image bearing members are each spaced from the belt member.
JP-A No. 2000-34031 discloses an image forming apparatus which controls the inclination of one of a plurality of support rotating bodies around which a belt member is wound and moves the belt member in the rotational axis direction of the belt member, whereby the wrinkling of the belt member is controlled. In this image forming apparatus, a deviation of the rotational position of the belt member is modified by performing such steering control that the belt member is moved in the longitudinal direction of the support rotating body in response to the output of a sensor for detecting a position in the rotational axis direction of the belt member.
In such steering control, for the purpose of detecting the position of an end of the belt member to adjust the position of the belt member, the stability of the shape of the end of the belt member is required. JP-A Nos. 7-334011, 11-219046, 2001-341883, and 2006-78612 propose various constitutions for preventing deformation of the end of the belt member. Specifically, there proposed a reinforced structure (JP-A No. 7-334011) and a rib (JP-A No. 11-219046) which prevent movement to the inside surface of the end of the belt member. JP-A No. 2001-341883 proposes a constitution for regulating a belt surface for the purpose of preventing a belt from being caught when detached and attached, on the ground of warpage of the belt of a roller portion, which serves as a fulcrum when the belt is swung. JP-A No. 2006-78612 proposes that the end of the belt member is nipped by using a pair of rollers. In addition to the above publications, JP-A Nos. 2009-42723 and 2008-309941 propose a constitution for preventing deformation of a belt.
As in JP-A No. 2000-34031, when the steering control is performed, the warpage of the end of the belt member is suppressed to thereby enhance the stability of the shape of the end of the belt member, whereby the stability of the control of the wrinkling of the belt member can be enhanced. Therefore, it is preferable that a fixed end regulating member is disposed on the upstream side of a detection device, which detects the position of the end of the belt member in the rotational direction of the belt member, whereby the warpage of the end of the belt member is regulated.
However, as in JP-A No. 2001-249519, when the belt member can be spaced from some image bearing members, the stretched surface of the belt member varies accompanying the contact/separation operation. Namely, in the full-color mode, for the purpose of preventing color deviation, it is preferable that the end of the belt member is regulated by the fixed end regulating member. Meanwhile, in the monochrome mode, the color deviation does not occur, and thus, if the end of the belt member is not regulated in the monochrome mode, the following problem occurs.
In the full-color mode, the wrinkling of the belt member is controlled in such a state that the warpage of the end of the belt member is small. Meanwhile, in the monochrome mode, the wrinkling of the belt member is controlled in such a state that the end of the belt member warps. Namely, in the monochrome mode, the position of the belt member is adjusted to the position of the state in which the end of the belt member warps. According to this constitution, when the mode is shifted from the monochrome mode to the full-color mode, the warpage of the end of the belt member is reduced, and therefore, the variation of the position of the end of the belt member detected by the detection device is increased. Consequently, the frequency of the wrinkling control for reducing the positional variation increases in the full-color mode, whereby the color deviation easily occurs.